In conventional electrohydraulic servo control systems, a pump is coupled through a plurality of pressure compensated flow control servo valves to a corresponding plurality of loads, such as hydraulic actuators or motors. In a typical application, the moving components of an earth excavator may be coupled to electrohydraulic actuators controlled by a master controller responsive to operator lever or joystick inputs. The pump is controlled to deliver an output pressure equal to the highest load pressure plus an incremental load sensing pressure drop, which is a function of servo valve design. Typically, a network of shuttle valves is coupled to the various loads and delivers an output to the pump control input indicative of highest load pressure. The pump is mechanically controlled to deliver an output pressure equal to such highest load pressure plus the load sensing pressure drop of the servo valve coupled to that load.
It has been proposed to eliminate the shuttle valve network, and to provide pump control by electronically sensing actual pressure at the various loads. Such a system requires pressure sensors at both of the servo valve ports at each load, a pressure sensor at the pump output, and a corresponding multiplicity of conductors for feeding the pressure sensor signals to an electronic pump controller. Although enhanced control characteristics can be obtained reduction in cost and complexity remains desirable.
It is therefore a general object of the present invention to provide an electrohydraulic servo system of the described character which exhibits reduced cost and complexity as compared with prior art systems of similar type. A further and more specific object of the invention is to provide an electrohydraulic servo system of the described type which includes pressure compensated flow control valves coupled to the various loads, and a pump controller responsive to flow demand at the several valves for controlling pump output to supply the sum of such demands. A further and yet more specific object of the invention is to provide an electrohydraulic system having a pump controller as described which automatically compensates for changes in pump input speed.